Quantitative genetic variation in declining plant populations

Sammanfattning: The ecological and genetic effects of habitat fragmentation are an issue of major concern in current conservation biology. Habitat fragmentation results in smaller habitat areas and as a consequence smaller and more isolated populations. Many habitats have become fragmented through human activities such as forestry and agricultural development. In Sweden, for example, the area of species-rich, semi-natural grasslands has decreased by more than 90% since the 1870s. The aim of this thesis was to examine the quantitative genetic effects of small population size and population fragmentation, with particular emphasis on the variation in evolutionarily relevant phenotypic characters. The thesis includes experimental studies of inbreeding depression and bottleneck effects as well as studies designed to reveal quantitative genetic effects of human-induced habitat fragmentation. The species used in the inbreeding/bottleneck experiment was Nigella degenii (Ranunculaceae) while Briza media (Poaceae) and Saxifraga granulata (Saxifragaceae) was used in the study of habitat fragmentation. Comparison of selfed and outbred offspring of Nigella revealed significant inbreeding depression in primary components of fitness but also in morphological and phenological characters that here diverged within the study species. The bottleneck experiment showed line-specific changes in genetic covariance structure but also indicated that bottlenecked populations generally have a lower adaptive potential than the ancestral population. There was no clear evidence that genetic drift, founder events and diversifying selection have played major roles in structuring the quantitative genetic variation within Briza in the study area. In contrast to the Briza study, several characters in Saxifraga were significantly correlated with habitat descriptors assumed to reflect selection pressures (e.g. current grazing pressure and percent cover of trees or shrubs). Again, no effect of drift and founder events were detected. The results for Briza and Saxifraga indicate that quantitative genetic effects of habitat fragmentation may be of lesser concern for conservation than more immediate effects on species diversity or short-term population viability.